The development of new battery chemistries, and particularly the commercial development of high capacity and high rate lithium cells, has increased the performance potential of both primary and secondary cells. These new cells tend to have more capacity per unit volume or weight (this is in part attributable to lithium being a light weight material with a very high capacity to weight ratio), or both. However, as a result of the chemistry involved, operating requirements of such cells must be more closely monitored and controlled, especially if rapid charging or discharging is needed.
Failure to observe safety precautions or to take into account different charging characteristics and requirements of such cells will, at the least, reduce the life of the battery and may cause it to overheat, vent, or leak electrolyte.
In addition, lithium batteries have the added potential for major physical damage because of the very high activity of the metal and some of its derivatives. Both users and manufacturers have already suffered extremely serious fires and explosions caused by various kinds of lithium cells, particularly under recharging conditions. Accordingly, charging conditions must be closely controlled to maintain both safety and optimal operation conditions.
Optimum performance of batteries in general is achieved when all cells in the battery are precisely balanced and are being charged and discharged in unison. Untoward events (leakage, venting, etc.) are avoided with such optimum performance conditions and safety is similarly not a problem. It is generally only when there is an imbalance in the system which cause the various untoward events. Though it is possible to overcompensate for the imbalances to ensure safety this is at a major cost of a reduction in cell capacity and performance.
In order to maintain optimum performance conditions it is important to initially determine normal discharge and charging conditions and then to provide means to only permit operation within the normal range of conditions. Thus, in the case of lithium secondary cells, at significant discharge rates, it is hazardous to discharge any cell below a level of about 2.25 volts and it is also necessary to limit charge voltage to 4.24.4 volts per cell, with the variation being a function of construction of the cell. Precise monitoring and shut off circuitry is necessary to avoid the detrimental low voltage discharge and high voltage charging conditions.
Because lithium ion (Li-Ion) cells get very hot under extended (several minutes) discharge at rates well above 1.5 C, it is desirable to be able to limit discharge accurately and predictably to maximize cell performance without the necessity of an excessive cell safety factor. This permits batteries to be designed to meet maximum pulse load requirements without requiring a large 'tolerance window whereby the point where over-current protection begins can be set only slightly higher than the highest specified operating current.
Normally, the charging protocol for Li-Ion batteries is relatively simple. The charger is set to charge at a limited maximum rate (depending upon the cell size and arrangement), which changes to fixed voltage when the battery approaches the desired maximum voltage per cell, A good quality laboratory power supply can do this quite easily, and many older chargers are adjustable to accomplish this as well.
If, however, a Li-Ion battery is placed on a charger which is set for a higher terminal voltage than 4.25V per cell, or a higher than rated current, there is a real possibility of fire or explosion. Further, if the battery is connected to a load above its current capability, it will overheat, even if the cells are not taken below the minimum voltage of 2.25 volts/cell.
Prior art integrated circuits which monitor the individual cell voltages and open-circuit the battery when any cell reaches maximum or minimum voltage are known and are currently marketed. And many of these devices also provide overcurrent protection. However, these existing monitoring circuits and devices have limitations when they are applied to relatively large, high-performance batteries such as lithium-ion polymer batteries.
A problem with the prior art battery protection integrated circuits (ICs) is that they measure current by measuring the voltage drop across the transistors used for power switching, with the IC being designed to switch OFF the battery at a predetermined number of millivolts. The transistors are typically MOSFETs which behave characteristically like resistors in the conductive mode with the millivolt level being determined thereby.
However, MOSFET resistance values tend to vary from sample to sample even within type; and different types vary widely. Additionally, resistance increases with the operating temperature of the MOSFET. Thus, with MOSFETs used in cut-off and control circuitry, the actual current limit will vary from battery to battery because of MOSFET variations. It will change more radically if the MOSFET type is changed, and it will decrease as the operating temperature increases. As a result, battery designers, faced with requirements to meet current drains which approach maximum for the cells, must allow for MOSFET variations (including expected temperature conditions of usage) and are therefore precluded from using the full capability of the cells.